The present invention relates to a memory circuit which is designed so that when employed in an output circuit of an indicating controller, for example, it is capable of yielding a desired analog signal for a long period of time and changing the analog signal upon request.
For instance, in industrial instruments various quantities being processed, such as flow rate, pressure, error and so forth, are each measured and the measured value and its preset value are provided to an indicating controller, which performs what is called a Proportional plus Integral plus Derivative operation (PID) to obtain proportional, integral and derivative values of a deviation between the measured and the preset values and outputs an operation signal by which the opening of a valve, fuel supply and so on are controlled so that the quantities being processed may follow the preset values. The abovesaid operation signal is usually an analog signal. Recently an indicating controller of the type employing a microcomputer has come into use. In this case, its output signal is in a digital form and converted by a D/A converter to an analog form and, for utilizing the microcomputer and the D/A converter for other various processing, too, the calculated operation signal is converted to an analog signal, which is sample-held in a sample hold circuit and its sample hold output is used as an operation signal.
A conventional sample hold circuit is formed by a holding capacitor and an output buffer. In order to prevent the analog output from fluctuation, it is necessary that the holding capacitor to be of a small leak current type and that the output buffer be of extremely high impedance. Since reduction of the leak current and enhancement of the input impedance are inevitably limited, however, fluctuation of the sampled output is unavoidable in a long use. It is also possible to adopt an arrangement that detects this output variation, i.e., the so-called drift, by the microcomputer and produces a digital operation signal in a manner to compensate for the drift, but when the microcomputer is out of order, the drift cannot be avoided. That is to say, although this kind of device is adapted so that not only an operation signal is automatically derived from the indicating controller but also an operation signal may be provided by a manual operation, it is unavoidable that the output drifts during manual operation. The above is generally pointed out in respect of the case of producing an analog signal as well as the indicating controller.